In electrophotographic recording techniques a "latent charge image" is produced on a photoconductor. This latent charge image is then developed by applying an electrostatically charged toner which is then transferred to, for example, paper, textiles, foils or plastic and is fixed by means, for example, of pressure, radiation, heat or the action of solvent. Typical toners are one- or two-component powder toners (also called one- or two-component developers), and there are also specialty toners, such as magnetic, liquid and polymerization toners, that are employed.
One measure of the quality of a toner is its specific charge q/m (charge per unit mass). In addition to the sign and level of the electrostatic charge, the principal, decisive quality criteria are the rapid attainment of the desired charge level and the constancy of this charge over a prolonged activation period. In addition to this, the insensitivity of the toner to climatic effects such as temperature and atmospheric humidity is a further important criterion for its suitability.
Both positively and negatively chargeable toners are used in photocopiers, laser printers, LEDs (light emitting diodes), LCS (liquid crystal shutter) printers or other digital printers based on electrophotographic techniques, depending on the type of process and type of apparatus.
To obtain electrophotographic toners or developers having either a positive or a negative charge it is common to add charge control agents. The chromophoric component employed in color toners typically comprises organic color pigments. Color pigments have considerable advantages over dyes owing to their insolubility in the application medium, examples of these advantages being better thermal stability and light fastness.
On the basis of the principle of subtractive color mixing, the three primary colors, yellow, cyan and magenta, can be used to reproduce the entire spectrum of color that is visible to the human eye. Exact color reproduction is possible only if the respective primary color meets the precisely defined coloristic requirements. Otherwise, it is not possible to reproduce some shades, and the color contrast is inadequate.
In full-color toners, in addition to the precisely defined coloristic requirements, the three toners yellow, cyan and magenta must also be matched exactly to one another in terms of their triboelectric properties, since they are transferred in succession in the same apparatus.
It is known that colorants may in some cases have a sustained effect on the triboelectric charge of toners. Because of the different triboelectric effects of colorants and the resulting effect, sometimes very pronounced, on toner chargeability, it is not possible simply to add the colorants to a toner base formulation made available at the start. On the contrary, it may be necessary to make available for each colorant an individual formulation to which the nature and the amount of the required charge control agent are tailored specifically. This procedure is, accordingly, laborious and, in the case of color toners for the three-color process, represents a further difficulty in addition to those already described above.
Another important practical requirement is that the colorants should have high thermal stability and good dispersibility. Typical temperatures at which colorants are incorporated into the toner resins, when using kneading equipment or extruders, are between 100.degree. C. and 200.degree. C. Correspondingly, thermal stability at 200.degree. C., and better still at 250.degree. C., is of great advantage. It is also important for the thermal stability to be assured over a relatively long period (about 30 minutes) and in a variety of binder systems. Typical toner binders are addition polymerization resins, polyaddition resins and polycondensation resins, such as styrene, styrene-acrylate, styrene-butadiene, acrylate, polyester, phenolic-epoxy resins, polysulfones and polyurethanes, individually or in combination, which may also include further components, such as charge control agents, waxes or flow assistants, or may have these components added subsequently.
The requirement is fundamentally for a yellow pigment which possesses very high transparency, greenishness, good dispersibility and, as far as possible, a neutral inherent triboelectric effect.
By a neutral inherent triboelectric effect is meant that the pigment exhibits as little effect as possible on the inherent electrostatic charge of the resin.
The transparency is of central importance since in full-color copying or printing the colors yellow, cyan and magenta are copied or printed on top of one another, the sequence of colors depending on the apparatus. If, then, an overlying color is not sufficiently transparent, the color below it is unable to show through to an adequate extent and the color reproduction is distorted. When copying or printing onto overhead sheets, the transparency is even more important, since a lack of transparency here even in only one color makes the entire projected image appear gray.